1. Field of the Invention
The present invention relates to image pickup tubes.
2. Description of the Prior Art
FIG. 10 is a sectional view of an image pickup tube in the prior art illustrating the construction adjacent the target surface. In FIG. 10, reference numeral 1 designates a glass bulb, numeral 2 a face plate, numeral 3 a target surface (photo conductive layer surface), numeral 4 indium for sealing, and numeral 5 a metal ring. Numeral 6 designates a signal carrying electrode which passes through the face plate 2 and contacts with the target surface 3. Numeral 7 designates a mesh electrode to correct the landing error of the electron beam. The mesh electrode 7 is mounted on a mesh holder 8 which is connected mechanically and electrically to the indium 4. Predetermined voltage is applied to the mesh electrode 7 through the metal ring 5, the indium 4 and the mesh holder 8.
In such an image pickup tube of the prior art, however, when forces are applied thereto from outside, so-called microphonic noise may be produced and a lateral stripe may appear on the screen surface. It is known that such microphonic noise is caused by vibration of the mesh electrode 7. That is, since the mesh electrode 7 is disposed near the target surface 3, if the mesh electrode 7 is vibrated, current is generated through capacitance between the mesh electrode 7 and the target surface 3 and introduced as noise in the signal current resulting in microphonic noise.
In order to reduce the microphonic noise, various methods to suppress vibration of the mesh electrode have been tried. However, the methods have problems in complicated structure high cost and insufficient effect.
It is believed that the reason microphonic noise is conspicuous in an image pickup tube of the prior art is that ince the rectangular opening of the mesh holder 8 has periphery of uniform shape, the mesh electrode 7 after recrystallization treatment at high temperature, i.e., so-called annealing treatment, has uniform tension in various portions, whereby a stable standing wave is generated during vibration, and attenuation of the vibration of the mesh electrode 7 is slow and the microphonic noise becomes conspicuous.